Fuel cells are noted as an environmentally friendly, clean power source. Typically, there are instances in which it is difficult to deal with load fluctuations with a single fuel cell, so there has been proposed a hybrid power supply system which combines a fuel cell and a secondary cell. With a hybrid power supply system, in specific conditions, namely, during fuel cell malfunction or startup, it is desirable to halt output by the fuel cell and supply power with only power from the secondary cell.
However, within a hybrid power supply system there was required a large-capacity switch in order to isolate the fuel cell from the system. Further, during connection of the fuel cell to the system, there occurred the problem of the possibility of excessive current flow from the fuel cell.